The invention relates to providing multiple sinusoidal voltage signals at the same frequency or simultaneously sweeping in frequency, each having a programmable amplitude and phase.
Several applications in structural dynamics testing and active vibration and acoustic control require multiple excitations programmed at specific amplitudes and phases. The excitations may be used to simulate rotational forces, excite particular vibration modes of a structure, or cancel unwanted vibration or noise. These multiple excitations require multiple channels of signal generation with programmable amplitude and phase.
A simple, prior art method of phase shifting a signal is to pass it through an analog circuit with an inductive and resistive load. The amount of phase the circuit provides could be controlled with a programmable resistor which changes the phase response of the circuit. The problem with this method is that the change in resistance also changes the amplitude response of the circuit. Therefore, this method has the disadvantage that only an arbitrary phase or amplitude can be commanded on each channel, but not both.
Another prior art method is where both amplitude and phase can be adjusted using a single-channel phase shifter. Multiple single-channel phase shifters can be referenced to the same input sinusoid to create multiple phased sinusoid outputs. The problem with this system is its lack of programmability. The user must manually adjust a gain and phase knob on each channel. At best, these adjustments would have to be made any time the user desired to change his test set-up with a different arrangement of amplitudes and phases on the channels. At worst, these adjustments would have to be made for every frequency change when controlling actuators with slightly different frequency responses. Each single phase shifter is programmable and can be controlled via standard GPIB instrument control. One disadvantage of this system is that each module can only adjust the phase and not the amplitude of each channel. Also, this system uses a programmable time delay to cause the phase shift. The user is required to manually input (via software) the time delay directly or the frequency of the input waveform. Therefore, the second disadvantage is that automatic phased frequency sweeps are not possible with this system and changing frequency manually to perform a sweep test would be cumbersome and time consuming.
Another prior art approach for producing programmable amplitude and phase-shifted sinusoids is to use multiple phase-locking commercial function generators. The disadvantage of this method is its expense for large numbers of channels such as those that might be needed to excite a bladed disk. (Some bladed disks have over 100 blades).
Therefore, there exists a need in the art for an inexpensive programmable multiple channel amplitude and phase shifter which can operate either at a single frequency or sweep in frequency. The traveling wave excitation system phase shifter chassis method and device of the invention is compact, inexpensive, and versatile when compared to customary methods for generating traveling wave excitation signals.